Fluid exchange in single venules will be measured by means of the micro occlusion technique. The systemic hematocrit will be artificially reduced and fluorescent 5 mu PVC spheres and fluorescein tagged red cells will be used as markers. Fluorescent dextran will be injected systemically and observed at the level of single microvessels by means of a silicon intensified target video camera. Extravascular diffusion and intravascular concentration of the fluorescent die will be utilized to determine the diffusion constant in tissue and the hydrodynamic conductivity of the vascular wall. These techniques will first be utilized in the mesentery and omentum in order to obtain base line data which can be compared with previous studies. Subsequently, the methods will be applied to a skeletal muscle preparation.